Electrostatic coating processes have been used to modify the surface characteristics of a substrate. In order to coat the substrate, a powder atomizer is combined with a feeder to deliver measured amounts of powder into an air stream. The air stream is directed to a coating apparatus, which electrically charges the powder particles so that they become attracted to the substrate. The powder is sometimes chemically highly reactive, and typically small in size. Strong electrostatic forces charge the powder particles and thereby cause them to be attached to the substrate. The substrate frequently is in continuous strip or web form, and advances continuously across through the coating apparatus.
Electrostatic forces can be extremely strong on small particles, equaling perhaps 10 to 1000 times their weight. The electrode is often placed 4 to 6 inches away from the substrate to permit the vast majority of the generated powder cloud to be diffused within that bound and thus beneficially influenced by the electrostatic effects. These include the electric field, ions created by the corona discharge energetically propelled by that field toward the strip, charge transfer by some of these ions colliding with the interspersed powder, and collision and momentum transfer between the energetic ions and the interspersed powder.
The powder dispensed from the powder feeder must be dispensed at uniform rates of flow, otherwise discontinuities or lack of uniformity may develop in the coating. The height of the powder within the powder feeder should be kept level, in order to maintain a uniform head pressure at the feeder inlet. Should the substrate be disposed above the powder feeder inlet, then the substrate cannot be more widely spaced therefrom because of the electrode placement. Maintaining and controlling the volume of powder within the powder feeder has been difficult because of the resulting limited and available height between the substrate and the feeder.
In order to evenly distribute the powder onto the substrate, the powder should be evenly distributed across the powder feeder. The discharge rate is determined by the amount of powder that must be provided per unit time to coat the substrate throughout its width to the desired thickness. Should the powder be non-uniformly distributed within the powder feeder, then the discharge rate from the powder feeder discharge will not be uniform. Non-uniform powder discharge from the feeder will result in discontinuous or non-uniform coatings. Thus, there is need in the art for an apparatus and method which functions to maintain a constant volume of powder throughout a powder feeder during operation of the electrostatic powder coater.
The inventors' attempts to solve the problem included shaking, blowing, levitating, and pushing the powder into the feeder. Shaking the powder along a transport path is disadvantageous, because an appropriate angle can not be achieved for adequate feeding of the powder along the range of discharge rates required to be attained and the strong tendency to agglomerate the powder. Blowing the powder into the powder feeder caused control over the amount of powder fed to the powder feeder to be lost, with the powder being non-uniformly distributed. Pushing the powder into the powder feeder may cause reactive powder to begin to onset chemical changes, so that the powder will agglomerate or sinter prior to discharge and/or prior to application to the substrate. The inventors also attempted to use a fluidization method to levitate powder in a slightly inclined trough through which the powder would flow laterally. This was not successful because of the required inclination angle, and the inability to place the powder uniformly across the relatively wide brush feeder hopper. Thus, there is a need in the art for an apparatus and method for maintaining a power feeder uniformly filled, while minimizing the tendency of the powder to react.